These studies are concerned with the identification of those viral genes which continue to be expressed by the cell-associated viral genome in cells that have been transformed by polyoma, SV40, or simian adenovirus 7 (SA-7). For this purpose transformed cell-dependent, host range viral mutants are being sought which are capable of growth on viral-transformed, but not normal permissive cells, presumably because they are complemented by viral genes of the cell-associated viral genomes with persistent expression in the transformed cell. Recent studies have shown that a polyoma mutant of this kind isolated by Benjamin is defective for induction of cellular DNA synthesis and abortive transformation in rat cells. Furthermore, it fails to protect cells from serum-less death. SV40 mutants of the transformed cell-dependent, host range type are also being sought. To date, a suitable SV40-transformed permissive cell has not been obtained. An alternative approach will take advantage of the ability of both SV40 and SA-7 to complement human adenoviruses on primate cells, suggesting that these viruses contain an analagous gene, and of the persistent expression of this function in SV40-transformed cells. Thus, we hope to isolate SV40 mutants able to grow on SA-7 transformed, but not on normal monkey cells, presumably because they are complemented by an SA-7 gene with persistent expression in SA-7 transformed cells which is analagous to a similar SV40 gene.